Nitrided niobium powders and niobium electrolytic capacitors

ABSTRACT

A nitrogen containing niobium powder is disclosed as well as electrolytic capacitors formed from the niobium powders. Methods to reduce DC leakage in a niobium anode are also disclosed.

BACKGROUND OF THE INVENTION

The present invention relates to nitrided niobium powders andelectrolytic capacitors using the nitrided niobium powders as well asmethods of making the powders and electrolytic capacitors.

For many years, it has been the goal of various researchers to developniobium electrolytic capacitors because of the high di-electric constantof its oxide and the relatively low cost of niobium compared to avariety of other metals. Initially, researchers in this field consideredthe possibility of using niobium as a substitute for tantalumcapacitors. Accordingly, many studies were conducted to determine thesuitability of replacing tantalum with niobium.

In some of these studies, however, it was concluded that niobium hasserious fundamental deficiencies that needed to be resolved, thusinferring that niobium was not an acceptable substitute for tantalum.(See J. Electrochem. Soc. p. 408 C, December 1977). In another study,one conclusion reached was that the use of niobium in solid electrolyticcapacitors seems very unlikely due to various physical and mechanicalproblems, such as field crystallization. (Electrocomponent Science andTechnology, Vol. 1, pp. 27-37 (1974)). Further, in another study, theresearchers concluded that anodically formed passive films on niobiumwere different from electrical properties accomplished with tantalum andthat the use of niobium led to complexities which were not present withtantalum. (See Elecrochimica Act., Vol. 40, no. 16, pp. 2623-26 (1995)).Thus, while there was initial hope that niobium might be a suitablereplacement for tantalum, the evidence showed that niobium was notcapable of replacing tantalum in the electrolytic capacitor market.

Besides tantalum electrolytic capacitors, there is a market for aluminumelectrolytic capacitors. However, the aluminum electrolytic capacitorshave dramatically different performance characteristics from tantalumelectrolytic capacitors.

A driving force in electronic circuitry today is the increasing movetoward lower Equivalent Series Resistance (ESR) and Equivalent SeriesInductance (ESL). As IC performance increases with submicron geometry,there is a need for lower power supply voltage and noise margin. At thesame time, increasing IC speeds require higher power needs. Theseconflicting requirements create a demand for better power management.This is being accomplished through distributed power supplies which needlarger currents for decoupling noise. Increasing IC speeds also meanlower switching times and higher current transients. The electricalcircuit must, therefore, also be designed to reduce the transient loadresponse. This broad range of requirements can be met if the circuit haslarge enough capacitance but low ESR and ESL.

Aluminum capacitors typically provide the largest capacitance of allcapacitor types. ESR decreases with increase in capacitance. Therefore,currently a large bank of high capacitance aluminum capacitors are usedto meet the above requirements. However, aluminum capacitors do notreally satisfy the designers' requirements of low ESR and ESL. Theirmechanical construction with liquid electrolyte inherently produce ESRin the 100s of milliohm along with high impedance.

SUMMARY OF THE INVENTION

A feature of the present invention is to provide nitrided niobiumpowders.

A further feature of the present invention is to provide nitridedniobium powders, preferably having high surface areas and physicalcharacteristics which permit the nitrided niobium powders to be formedinto a capacitor having high capacitance.

Another feature of the present invention is to provide nitrided niobiumpowders which, when formed into capacitors, have a low DC leakage.

An additional feature of the present invention is to provide a method ofreducing the DC leakage in a capacitor formed from nitrided niobiumpowder.

Additional features and advantages of the present invention will be setforth in part in the description which follows, and in part will beapparent from the description, or may be learned by practice of thepresent invention.

The present invention relates to a nitrided niobium powder. Anotheraspect of the present invention relates to any nitrided niobium powderhaving a BET surface area of at least about 0.15 m² /g.

The present invention also relates to a nitrided niobium powder, whichwhen formed into an electrolytic capacitor anode, the anode has acapacitance of 30,000 CV/g to about 61,000 CV/g.

Also, the present invention relates to a method to reduce DC leakage ina niobium anode made from nitrided niobium powder which comprises thestep of introducing into the niobium powder a sufficient amount ofnitrogen to reduce the DC leakage in a capacitor anode that is formed.

It is to be understood that both the foregoing general description andthe following detailed description are exemplary and explanatory onlyand are intended to provide further explanation of the presentinvention, as claimed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a graph showing the BET surface areas of niobium powders andtheir respective capacitance when formed into anodes and sintered at atemperature of 1750° C.

FIG. 2 is a graph depicting the BET surface areas of niobium powders andtheir respective capacitance when formed into anodes and sintered at atemperature of 1600° C.

FIG. 3 is a graph depicting the BET surface areas of a niobium powdersand their respective capacitance when formed into anodes and sintered ata temperature of 1450° C.

FIG. 4 is a graph depicting the BET surface areas of niobium powders andtheir respective capacitance when formed into anodes and sintered at atemperature of 1300° C.

FIG. 5 is a graph showing various sintering temperatures of niobiumanodes and their respective calculated maximum capacitance.

FIG. 6 is a graph depicting the oxygen doping content of niobium powdersas well as their respective DC leakage when formed into anodes andsintered at different temperatures and using a forming voltage of 50volts.

FIG. 7 is a graph showing niobium powders having various doping levelsof oxygen as well as the respective DC leakage when formed into anodesand sintered at various temperatures and using a forming voltage of 30volts.

FIG. 8 is a graph showing the effects of various levels of phosphorusdoping in niobium powders and their respective capacitance when formedinto anodes.

FIG. 9 is a graph showing the effects of various phosphorus dopinglevels of niobium powder and their respective DC leakage when formedinto anodes.

FIG. 10 is a graph showing the amount of nitrogen present in variousniobium powders and their respective DC leakage when formed into anodesand sintered at 1300° C. or 1450° C. using a forming voltage of 50volts.

FIG. 11 is a graph showing the amount of nitrogen present in variousniobium powders and their respective DC leakage when formed into anodesand sintered at 1300° C. or 1450° C., using a forming voltage of 35volts.

FIG. 12 is a graph showing the same niobium samples of FIG. 10 withregard to oxygen content and DC leakage.

FIG. 13 is a graph showing the same niobium samples of FIG. 11 withregard to oxygen content and DC leakage.

FIG. 14 is a graph showing the amount of nitrogen present in variousniobium powders and their respective DC leakage when formed into anodesand sintered at 1300° C. using a forming voltage of 35 volts.

FIG. 15 is a graph showing the same niobium samples of FIG. 14 withregard to oxygen content and DC leakage.

FIG. 16 is a graph showing the amount of nitrogen present in variousniobium powders and their respective DC leakage when formed into anodesand sintered at 1450° C., using a forming voltage of 35 volts.

FIG. 17 is a graph showing the same niobium samples of FIG. 16 withregard to oxygen content and DC leakage.

FIG. 18 is a graph showing the amount of nitrogen present in variousniobium powders and their respective DC leakage when formed into anodesand sintered at 1300° C. using a forming voltage of 50 volts.

FIG. 19 is a graph showing the same niobium samples of FIG. 18 withregard to oxygen content and DC leakage.

FIG. 20 is a graph showing the amount of nitrogen present in variousniobium powders and their respective DC leakage when formed into anodesand sintered at 1450° C. using a forming voltage of 50 volts.

FIG. 21 is a graph showing the same niobium samples of FIG. 20 withregard to oxygen content and DC leakage.

DETAILED DESCRIPTION OF THE PRESENT INVENTION

The present invention relates to niobium powders having nitrogencontained therein. The amount of nitrogen present is generally greaterthan nitrogen amounts found in niobium powders as impurities. Themajority of the nitrogen present in the niobium powders of thisembodiment is a result of intentional conditions which lead to increasedlevels of nitrogen in the niobium powders (i.e., nitriding of theniobium). The nitrogen present in the niobium can be accomplished in anymanner. For instance, the nitrogen can be introduced (e.g., doped) intothe niobium during any processing stage of the niobium, such as duringone or more of the following stages: melting of a niobium ingot; adeoxidation stage; hydriding of the niobium; delubing of the niobium;any sintering of the niobium (e.g., such as sintering of the niobiumcapacitor anode); any thermal processing of the niobium; any heattreatment stage; or anytime before or after any one or more of theseprocessing steps or stages.

Any means can be used to nitride the niobium material, such as, but notlimited to, exposure to nitrogen containing environments (e.g., N2 gas)or nitrogen-containing materials, preferably during a thermal cycling todefuse the nitrogen in the material (e.g., preparing a solid-solution ofnitrogen by reaction of nitrogen containing materials by diffusion fromdirect physical contact or gas adsorption and/or absorption.).

One of the benefits of nitrogen in the niobium is a decrease in DCleakage for capacitors made at least in part from the niobium powder.The amount of nitrogen in the niobium can be any amount and preferablyamounts which do not lead to detrimental properties of the niobiumpowder or articles made from the niobium powder. Preferred amounts ofthe nitrogen are amounts which reduce the DC leakage of capacitors madefrom niobium powders. Some of the examples show that there is a point ofdiminishing return and that at certain levels of nitrogen, there is noadded benefit with respect to reducing DC leakage. Generally, the amountof nitrogen present is at least about 300 ppm, and can be at least about400 ppm or at least about 500 ppm or higher. The nitrogen range can befrom about 300 ppm to about 5,000 ppm and other ranges can be from about500 ppm to about 4,000 ppm; from about 500 ppm to about 3,500 ppm; 1,500ppm to about 5,000 ppm; and/or from about 500 ppm to about 3,000 ppmnitrogen.

With respect to DC leakage of capacitors made from the nitrided niobium,various decreases of the DC leakage can be observed. Generally,decreases of about 50% or less, or 25% or less, can be achieved bynitriding the niobium, compared to niobium powder having substantiallyno nitrogen (e.g., less than 100 ppm nitrogen).

The amount of nitrogen in the niobium powder and the benefits achievedfrom the presence of nitrogen can be based in part on the formingvoltage of the capacitor as well as the sintering temperature of theniobium powder. Preferably the sintering temperature is from about 1200°C. to about 1750° C., and more preferably from about 1300° C. to about1600° C. Generally, the higher the sintering temperature, the lower theDC leakage. Accordingly, high sintering temperatures are preferred.Also, for purposes of this embodiment, the forming voltage of thecapacitor made from the niobium powder with nitrogen can be any formingvoltage suitable for capacitors made in part from niobium such as about50 volts or less or ranges from about 30 volts to about 50 volts. Otherranges of forming voltages can be from about 30 to about 35 volts orless (25, 20, 15, 10, 5 volts) since lower forming voltages can alsocontribute to lowering DC leakage.

The niobium that can be used in this embodiment is any niobium powder,such as flaked, angular, nodular, and mixtures or variations thereof.Any of the embodiments set forth and/or described below can also besubjected to conditions which will lead to niobium powders having theabove-described nitrogen amounts.

With respect to the flaked niobium powder, the flaked niobium powder canbe characterized as flat, plate shaped, and/or platelet. Also, theflaked niobium powder can have an aspect ratio (ratio of diameter tothickness) of from about 3 to about 300, and preferably, from about 3 toabout 30. The flaked niobium powder permits enhanced surface area due toits morphology. Preferably, the BET surface area of the flaked niobiumpowder is at least 0.15 m² /g and more preferably, is at least about 1.0m² /g and even more preferably, is at least about 2.0 m² /g. Preferredranges of BET surface area for the flaked niobium powder are from about1.0 m² /g to about 5.0 m² /g and more preferably from about 2.0 m² /g toabout 5.0 m² /g or from about 2.0 m² /g to about 4.0 m² /g. The BETranges are based on pre-agglomerated flaked niobium powders.

The flaked niobium powder can be agglomerated. The flaked niobium powdercan also be hydrided or non-hydrided. The agglomerated flaked niobiumpowder preferably has a Scott Density of less than about 35 g/in³, andmore preferably about 10 to about 35. The unagglomerated flaked niobiumpowder preferably has a Scott Density of less than about 12, and morepreferably, less than about 5 g/in³. Preferably, the agglomerated flakedniobium powder has a flow of greater than 80 mg/s, more preferably fromabout 80 mg/s to about 500 mg/s.

The flaked niobium powder can be prepared by taking a niobium ingot andmaking the ingot brittle by subjecting it to hydrogen gas for hydriding.The hydrided ingot can then be crushed into an angular powder, forinstance, with the use of a jaw crusher. The hydrogen can then beremoved by heating in a vacuum and the degassed angular powder can thenbe subjected to milling, such as with use of a stirred ball mill wherethe powder is dispersed in a fluid medium (aqueous or non-aqueous) suchas ethanol, to form the flaked powder by the impact of the stainlesssteel balls moved by the action of rotating bars. Various sizes offlakes can be made by hydrogen embrittlement followed by subjecting theflakes to impact milling, for example with use of a fluidized bed jetmill, Vortec milling, or other suitable milling steps.

The flaked niobium powder can optionally have a high oxygen content,such as by doping or other oxygen introduction methods. The amount ofoxygen doping content can be at least about 2,000 ppm. More preferably,the flaked niobium powder has an oxygen content of from about 2,000 ppmto about 20,000 ppm and more preferably from about 2,750 ppm to about10,000 ppm, and most preferably from about 4,000 ppm to about 9,000 ppm.The doping of the niobium powder with oxygen can be done in a variety ofways including, but not limited to, repeated heating in vacuum at 900°C. and cooling in air. Alternatively, the flaked niobium or any othertype of niobium can have a low oxygen content, such as less than 1,000ppm.

Further, the flaked niobium powder can be also doped with phosphorusalone or with oxygen. The doping of the niobium powder with phosphorusis also optional. In one embodiment of the present invention, the amountof phosphorus doping of the niobium powder is less than about 400 ppmand more preferably less than about 100 ppm, and most preferably lessthan about 25 ppm.

Based on an example set forth herein, the amount of phosphorus dopingcan be unimportant with respect to the DC leakage and capacitance of ananode formed from a niobium powder having various levels of phosphorusas a dopant. Accordingly, in one embodiment, low amounts of phosphorusand even negligible amounts or no phosphorus is present since phosphoruscan have small or no benefits for DC leakage and capacitance withrespect to certain anodes formed from niobium powders.

Other examples of niobium powders include niobium powder (e.g., flaked,angular, nodular, and mixtures thereof) having a significant level ofoxygen present in the niobium powder. The oxygen level can be achievedin the same manner as described above. Preferably, the amount of oxygenin the niobium powder is at least about 2,000 ppm, and more preferablyfrom about 2,000 ppm to about 20,000 ppm. Other preferred ranges ofoxygen content in niobium powder are from about 2,750 ppm to about10,000 ppm and levels of from about 4,000 ppm to about 9,000 ppm. Withrespect to these niobium powders, like the embodiment relating to theflaked niobium powder only, the phosphorus levels in the niobium powderscan be considerably low for certain embodiments. Preferably, in suchembodiments, the phosphorus level (as a dopant) is less than about 400ppm and more preferably less than about 100 ppm, and most preferablyless than about 25 ppm.

Another example of niobium powders are niobium powders (e.g. flaked,angular, nodular, and mixtures thereof) having a BET surface area of atleast 0.5 m² /g and preferably, at least about 1.0 m² /g, and morepreferably from about 1.0 to about 5.0 m² /g, and most preferably fromabout 2.0 to about 5.0 m² /g. The BET ranges are based onpre-agglomerated niobium powders. The niobium powder can be hydrided ornon-hydrided. Also, the niobium powder can be agglomerated. The niobiumpowder in this embodiment can be doped with nitrogen. Also, for certainuses, the niobium powder can have an oxygen content below about 2,000ppm.

With respect to making the flaked niobium powder or the niobium powderhaving any morphology with the BET surface area, the examples show thepreferred steps of forming the niobium powder which can thensubsequently be made into a flake or other morphology. In general, theprocess is as follows and the examples provide specific details as topreferred embodiments of making the niobium powders of the presentinvention.

Generally, in preparing the niobium powders having a BET surface area ofat least 0.5 m² /g, a niobium ingot is hydrided by heating in a vacuumto form an embrittled ingot which is crushed into a powder. The hydrogenin the powders can optionally be removed by heating the particle in avacuum. The various BET surface areas can be achieved by subjecting thepowder to milling, preferably an attritor milling process. The higherthe BET surface area of the powder generally will require a longermilling time. For instance, with a milling time of approximately 60minutes a BET surface area of approximately 1.0 m² /g can be achieved.To obtain even higher BET surface areas, longer milling times will beneeded and to achieve the BET surface area of from about 4 to about 5 m²/g or greater, milling times on the order of approximately 24 hours ormore in an attritor mill is one way of making such niobium powdershaving high BET surface area ranges. When making such high surfaceareas, it is preferred to use a 30-SL Union Process attritor mill using1,000 lbs 3/16" SS media, and approximately 80 pounds of niobium powderwith the mill set at a rotation of approximately 130 rpm. Also, the millwill contain a sufficient amount of a medium such as ethanol on theorder of 13 or more gallons. After milling, the niobium powders are thensubjected to a heat treatment and preferably the niobium powders canhave a phosphorus content to help in minimizing the reduction in surfacearea during the heat treatment. The heat treatment can be anytemperature sufficient to generally cause agglomeration and preferablywithout reducing the surface area. A temperature for heat treatmentwhich can be used is approximately 1100° C. for 30 minutes. However thetemperature and time can be modified to ensure that the high BET surfacearea is not reduced.

The various niobium powders described above can be further characterizedby the electrical properties resulting from the formation of a capacitorusing the niobium powders of the present invention. In general, theniobium powders of the present invention can be tested for electricalproperties by pressing the niobium powder into an anode and sinteringthe pressed niobium powder at appropriate temperatures and thenanodizing the anode to produce an electrolytic capacitor anode which canthen be subsequently tested for electrical properties.

Accordingly, another embodiment of the present invention relates tocapacitors formed from the nitrogen containing niobium powders of thepresent invention. Anodes made from some of the niobium powders of thepresent invention can have a capacitance of from 30,000 CV/g to about61,000 CV/g. In forming the capacitor anodes of the present invention, asintering temperature is used which will permit the formation of acapacitor anode having the desired properties. Preferably, the sinteringtemperature is from about 1200° C. to about 1750° C., and morepreferably from about 1200° C. to about 1400° C., and most preferablyfrom about 1250° C. to about 1350° C.

The anodes formed from the niobium powders of the present invention arepreferably formed at a voltage of less than about 60 volts, andpreferably from about 30 to about 50 volts and more preferably at about40 volts. Lower forming voltages are also possible, such as about 30volts or less. Preferably, the working voltages of anodes formed fromthe niobium powders of the present invention are from about 4 to about16 volts and more preferably from about 4 to about 10 volts. Also, theanodes formed from the niobium powders of the present inventionpreferably have a DC leakage of less than about 5.0 na/CV. In anembodiment of the present invention, the anodes formed from some of theniobium powders of the present invention have a DC leakage of from about5.0 na/CV to about 0.50 na/CV.

The present invention also relates to a capacitor in accordance with thepresent invention having a niobium oxide film on the surface thereof.Preferably, the niobium oxide film comprises a niobium pentoxide film.

The capacitors of the present invention can be used in a variety of enduses such as automotive electronics; cellular phones; computers, such asmonitors, mother boards, and the like; consumer electronics includingTVs and CRTs; printers/copiers; power supplies; modems; computernotebooks; and disk drives.

The present invention will be further clarified by the followingexamples, which are intended to be exemplary of the invention.

    ______________________________________                                        TEST METHODS                                                                  ______________________________________                                        Anode Fabrication:                                                            size - 0.197" dia                                                             3.5 Dp                                                                        powder wt = 341 mg                                                            Anode Sintering:                                                              1300 Deg C.* 10'                                                              1450 Deg C.* 10'                                                              1600 Deg C.* 10'                                                              1750 Deg C.* 10'                                                              30 V Ef Anodization:                                                          30 V Ef @ 60 Deg C./0.1% H.sub.3 PO.sub.4 Electrolyte                         20 mA/g constant current                                                      DC Leakage/Capacitance - ESR Testing:                                         DC Leakage Testing - - -                                                      70% Ef (21 VDC) Test Voltage                                                  60 second charge time                                                         10% H.sub.3 PO.sub.4 @ 21 Deg C.                                              Capacitance - DF Testing:                                                     18% H.sub.2 SO.sub.4 @ 21 Deg C.                                              120 Hz                                                                        50 V Ef Reform Anodization:                                                   50 V Ef @ 60 Deg C./0.1% H.sub.3 PO.sub.4 Electrolyte                         20 mA/g constant current                                                      DC Leakage/Capacitance - ESR Testing:                                         DC leakage Testing - - -                                                      70% Ef (35 VDC) Test Voltage                                                  60 second charge time                                                         10% H.sub.3 PO.sub.4 @ 21 Deg C.                                              Capacitance - DF Testing:                                                     18% H.sub.2 SO.sub.4 @ 21 Deg C.                                              120 Hz                                                                        75 V Ef Reform Anodization:                                                   75 V Ef @ 60 Deg C./0.1% H.sub.3 PO.sub.4 Electrolyte                         20 mA/g constant current                                                      DC Leakage/Capacitance - ESR Testing:                                         DC leakage Testing                                                            70% Ef (52.5 VDC) Test Voltage                                                60 second charge time                                                         10% H.sub.3 PO.sub.4 @ 21 Deg C.                                              Capacitance - DF Testing:                                                     18% H.sub.2 SO.sub.4 @ 21 Deg C.                                              120 Hz                                                                        ______________________________________                                    

Scott Density, oxygen analysis, phosphorus analysis, and BET analysiswere determined according to the procedures set forth in U.S. Pat. Nos.5,011,742; 4,960,471; and 4,964,906, all incorporated hereby in theirentireties by reference herein.

EXAMPLE 1

This example illustrates an embodiment of this invention comprisingangular niobium powder. Electron beam produced niobium ingot washydrided by heating the ingot in a vacuum of 10⁻⁴ torr to 850° C. for120 minutes. The vacuum was replaced by hydrogen gas purge at 21 kPa forsufficient time to embrittle the ingot. The vacuum was then pumped downto -28" mercury and then backfilled with argon to -5" Hg. The pressurewas maintained until the temperature, as measured by a workthermocouple, stabilized. Air was gradually introduced in increasingpressure such that the work temperature did not rise. The embrittledingot was crushed into angular powder in a jaw crusher and classified byextracting powder which passed through a No. 325 sieve screen(equivalent to a 44 micrometer particle size). Hydrogen was removed fromthe size-reduced hydrogen-containing particles by heating the particlesto 850° C. in a vacuum until pressure was no longer affected by hydrogenbeing emitted from the particles to provide niobium metal angular powderhaving a Fisher Sub Sieve Size of 10.6 micrometers, a Scott density of2.67 g/cc (43.8 g/in³), a pre-agglomerated BET surface area of 0.17 m²/g and 1770 ppm oxygen; the ratio of oxygen to BET surface area was10,400 ppm O/m² /g, and the flow was 19 mg/sec. About 0.34 g samples ofunagglomerated angular niobium powder were pressed into an anode mold 5mm in diameter around a 0.6 mm diameter niobium lead wire to a densityof 3.5 g/cc. Samples of the pressed niobium powder were sintered in avacuum (at less than 10⁻³ Pa) at four different temperatures, i.e. 1300,1450, 1600 and 1750° C. for 10 minutes, then anodized by applying 20mA/g constant current at 50 V to the anode immersed in 0.1 weightpercent phosphoric acid to produce electrolytic capacitor anodes, whichwere washed and dried. The capacitor performance characteristics,evaluated by measurements on the anodes immersed in 18 wt % sulfuricacid, are reported in Table 1. Capacitance, determined at a frequency of120 Hertz, is reported in units of microfarad volts per gram (CV/g) andmicrofarad volts per cubit centimeter of anode volume (CV/cc); DCleakage, measured after a 1 minute charge of 35 volts, is reported inunits of nanoamperes per microfarad-volt (nA/CV).

EXAMPLE 2

This example illustrates an embodiment of the powder of this inventioncomprising agglomerated mixture of angular and flaked powder. 2.5 lbs ofdegassed angular powder prepared essentially in the manner of Example 1was processed in a 1-S Union Process attritor stirred ball mill (285 rpmfor 90 minutes) where powder dispersed in 2,400 ml ethanol medium and 40lbs 3/16" 440SS medium was formed into flaked powder by the impact ofstainless steel balls moved by the action of rotating bars. After thedesired deformation into flake, the niobium powder was then removed andwashed to remove any alcohol present. The niobium powder was then washedwith a mixture of deionized water, hydrofluoric acid and hydrochloricacid in an amount of 500 ml/lb, 4 ml/lb and 250 ml/lb of niobiumrespectively (18.6% HCl containing 22 ml/kg HF) to remove metalcontamination (e.g. iron, nickel, chromium and the like transferred fromcontact with stainless steel balls). Afterwards, the niobium powder wasagain washed with deionized water and then dried. The acid washed flakedpowder was dried in air at 85° F. (30° C.) and had an aspect ratio(determined by observation of micrographs) in the range of 50 to 70. Theflaked powder was blended with starting angular powder (in the weightratio of 30:70) and with a phosphorus containing powder, i.e. NH₄ PF₆,in an amount to provide 60 ppm phosphorus which serves as a grainretarding agent to minimize the reduction in surface area duringsubsequent heat treatment for agglomeration. The pre-agglomerated BETsurface area was 0.31 m² /g. The mixed powders were agglomerated byheating in a vacuum at 1100° C. for 30 minutes to form an agglomeratedmass. The agglomeration procedure was performed in a manner such thatthe material was pumped down to a high vacuum and heated at a ramp rateof 3° C./minute to 700° C. and held for outgassing until the highpressure was achieved. The heating continued in the furnace at a ramprate of 8° C./minute to 1100° C. under high pressure and held for 30minutes. The material was then allowed to cool in the furnace and thematerial was manually passivated by exposing it to air. The material wasthen reduced to smaller agglomerated particles by jaw crushing; reducedparticles passing a No. 50 sieve size (equivalent to a maximumagglomerated particle size of 300 micrometers) exhibited a Scott densityof 1.3 g/cc (21.7 g/in³), a BET surface area of 0.26 m² /g, oxygencontent of 3693 ppm and phosphorus content of 25 ppm; the ratio ofoxygen to BET surface area was 14,000 ppm O/m² /g and a flow of 22mg/sec. The agglomerated powder was fabricated into anodes and testedfor electrical properties in the manner of Example 1 which are reportedin the Table 1.

EXAMPLE 3

This example illustrates an embodiment of the powder of this inventioncomprising agglomerated flaked powder. Acid leached flaked powder havingan aspect ratio of about 50 to 70 was prepared essentially as describedin Example 2 (cycle time of 60 minutes) except the niobium powder washydrided a second time by exposure to hydrogen at 20.7 kPa (3 psig) and850° C. to provide an embrittled flake which was cooled and reduced insize by self impaction in a fluidized bed Jet mill (obtained fromHosokawa Micron Powder Systems, Summit, N.J.) to make flaked powderhaving a median particle size of 6 micrometers (as determined by laserparticle size scanning). The pre-agglomerated BET surface area was 0.62m² /g. The reduced-size flaked powder was agglomerated by heating in ahydrogen atmosphere by heating the furnace at a rate of 10° C./minute to1050° C. under a vacuum furnace and holding this temperature until thefurnace pressure decreased below 100 microns. Tantalum coarse chips(10-20 mesh) were used as an oxygen getter in a weight ratio of 1 Nb to1-1.5 Ta. The furnace was then backfilled with hydrogen to obtain apressure of 360 mmHg and the furnace temperature was then increased to1200° C. and held for 1 hour. The hydrogen was then evacuated until thefurnace pressure decreased to less than 1 micron and the furnace wasallowed to cool to room temperature. The niobium powder was thenpassivated in air for 30 cycles wherein the operating pressure wasincreased by 20 torr for each cycle and held for 2 minutes beforestarting the next backfill of air. The agglomerated niobium powder wasreduced in size to agglomerated particles by a jaw crusher; reducedagglomerated flaked niobium powder was separated by screening through aNo. 50 sieve size screen (equivalent to a maximum agglomerated flakedparticle size of 300 micrometers) and exhibited a Scott density of 1.21g/cc (20.4 g/in³), a BET surface area of 0.46 m² /g, oxygen content of8760 ppm; the ratio of oxygen to BET surface area was 19,000 ppm O/M²/g, and a flow of less then 1 mg/sec. The agglomerated powder wasfabricated into anodes and tested for electrical properties in themanner of Example 1 and reported in Table 1.

EXAMPLE 4

This example illustrates another embodiment of the powder of thisinvention comprising high surface area, low oxygen, agglomerated niobiumflaked powder. Niobium powder was prepared in the same manner as inExample 3 except the niobium powder was attritor milled for 90 minutes,and heat treatment was carried out in a vacuum at 1150° C. for 30minutes. The pre-agglomerated BET surface area was 0.85 m² /g. Theoxygen content of quantities of flaked niobium powder preparedessentially in the manner of Example 3 was reduced by heating niobiumpowder admixed with 4 to 5 wt % magnesium powder under argon at atemperature in the range of 750 to 850° C. for 2 hours. The magnesiumcontent was established as being in the range of 2 to 3 times thestoichiometric amount of oxygen in the niobium powder. After cooling,residual magnesium and oxides were removed from agglomerated flakedniobium by nitric acid leach. Deoxidized flaked niobium was waterwashed, dried, and separated by screening through a No. 50 sieve sizescreen. The screened niobium flake exhibited a Scott density of 1.47g/cc (24.1 g/in³), a BET surface area of 0.96 m² /g, an oxygen contentof 3130 ppm; the ratio of oxygen to BET surface area was 3260 ppm O/m²/g, and a flow of 76 mg/sec. The agglomerated powder was fabricated intoanodes and tested for electrical properties in the manner of Example 1,and reported in Table 1.

                  TABLE 1                                                         ______________________________________                                                Sinter temperature                                                            1300° C.                                                                       1450° C.                                                                         1600° C.                                                                        1750° C.                            ______________________________________                                        Example 1:                                                                    Capacitance                                                                   (CV/g)    8400      7500      6400   5500                                     (CV/cc)   40900     37000     33400  30000                                    DC Leakage                                                                              53        2.8       2.3    2.4                                      (na/CV)                                                                       Sinter Density                                                                          4.9       5.0       5.2    5.5                                      (g/cc)                                                                        Example 2:                                                                    Capacitance                                                                   (CV/g)    13600     11900     10000  8200                                     (CV/cc)   46000     41600     36900  33400                                    DC Leakage                                                                               25       1.7       2.1    2.5                                      (na/CV)                                                                       Sinter Density                                                                           3.4      3.5       3.7    4.1                                      (g/cc)                                                                        Example 3:                                                                    Capacitance                                                                   (CV/g)    32500     21400     13400  7100                                     (CV/cc)   114100    94300     73600  45800                                    DC Leakage                                                                              5.8       4.1       2.4    2.0                                      (na/CV)                                                                       Sinter Density                                                                          3.5       4.4       5.5    6.4                                      (g/cc)                                                                        Example 4:                                                                    Capacitance                                                                   (CV/g)    31,589    21,059    12,956 7,254                                    (CV/cc)   110,562   88,448    64,780 42,799                                   DC Leakage                                                                              5.8       5.3       2.6    1.4                                      (na/CV)                                                                       Sinter Density                                                                          3.5       4.2       5.0    5.9                                      (g/cc)                                                                        ______________________________________                                    

EXAMPLE 5

A niobium powder was prepared in the same manner as in Example 4 exceptthe heat treatment occurred in a vacuum at 1250° C. for 30 minutes. Thepre-agglomerated BET surface area was 0.78 m² /g. The powder was formedinto an anode as in Example 1 and had the following performancecharacteristics

    ______________________________________                                        Cv/g @ 50 Vf  19,600 (1450° C.)                                                                   31,040 (1300° C.)                           Sinter Density, g/cc                                                                        4.8 (1450° C.)                                           DC Leakage, na/Cv                                                                           2.33 (1450° C.)                                          BET, m.sup.2 /g                                                                             0.80                                                            Oxygen, ppm   2,815                                                           Scott Density, G/in.sup.3                                                                   24.0                                                            Flow, mg/sec  97                                                              ______________________________________                                    

EXAMPLE 6

A niobium powder was prepared in the same manner as in Example 4 exceptthe niobium powder was in an attritor mill for 150 minutes and the heattreatment was in a vacuum furnace where the pressure was pumped down to1 micron and then the temperature was increased by 50° C./minute to 950°C. and held until the high vacuum was achieved. The temperature was thenincreased by 15° C. stages until a temperature of 1250° C. was reachedand that temperature was held for 30 minutes. The material was thenallowed to cool to room temperature under vacuum and passivated for 30cycles, wherein the pressure was increased by 20 torr after each cycleand held for 2 minutes before starting the next backfill of air. Thepowder was then crushed in a -50 mesh jaw crusher and deoxidized byblending the powder with 4% w/w magnesium metal and placing the materialin a retort furnace and pumping down to 100 microns. Thepre-agglomerated BET surface area was 1.05 m² /g. The furnace was thenbackfilled with argon to a pressure of 800 torr and the pressureincreased to 800° C. and held for 2 hours. The material was then allowedto cool to room temperature and passivated in air for 30 cycles in thesame manner mentioned above in Example 3. The material was then washedwith a mixture of deionized water (500 ml/lb), hydrofluoric acid (4ml/lb) and nitric acid (250 ml/lb). The powder was then rinsed withdeionized water and dried. The niobium powder was then formed into ananode as in Example 1 and had the following performance characteristics

    ______________________________________                                        CV/g @ 50 Vf (Sintering Temp.)                                                                 24,300 (1450° C.)                                                                  41,700 (1300° C.)                         Sinter Density, g/cc                                                                           4.0 (1450° C.)                                        DC Leakage, na/Cv                                                                              1.5 (1450° C.)                                        BET, m.sup.2 /g  1.11                                                         Oxygen, ppm      3,738                                                        Scott Density, g/in.sup.3                                                                      24.4                                                         Flow, mg/sec     112                                                          ______________________________________                                    

EXAMPLE 7

Niobium powder was prepared in the same manner as in Example 6 exceptthe niobium powder was blended with phosphorus before heat treatment toachieve a phosphorus loading of 56 ppm. The pre-agglomerated BET surfacearea was 1.05 m² /g. The material was hydrided as in Example 3 andcrushed, heat treated, and deoxidized as in Example 6. The niobiumpowder was then formed into an anode as in Example 1 and had thefollowing performance characteristics

    ______________________________________                                        Cv/g @ 50 Vf (Sintering Temp.)                                                                 29,900 (1450° C.)                                                                  45,400 (1300° C.)                         Sinter Density, g/cc                                                                           3.7 (1450° C.)                                        DC Leakage, na/Cv                                                                              1.3 (1450° C.)                                        BET, m.sup.2 /g  1.07                                                         Oxygen, ppm      3,690                                                        Scott Density, g/in.sup.3                                                                      23.2                                                         Flow, mg/sec     76                                                           ______________________________________                                    

EXAMPLE 8

Niobium powder was prepared in the same manner as in Example 4 exceptthe niobium powder was milled in a 30 S attritor mill (130 rpm) for 8hours by using 1,000 lbs of 3/16" SS media, 80 lbs of niobium powder,and 13 gallons of ethanol. The milled powder was acid leached and washedin the same manner as described before and the material had thefollowing characteristics

    ______________________________________                                        BET, m.sup.2 /g  1.39                                                         Oxygen, ppm      8,124                                                        Scott Density, g/in.sup.3                                                                      3                                                            ______________________________________                                    

The niobium powders of examples 1-8 were discovered to have nitrogenlevels on the order of about 1,000 ppm to about 2,000 ppm N2 as a resultof air leaking into a furnace during the degassing of the Nb ingot chip.

EXAMPLE 9

FIGS. 1, 2, 3, and 4 show CV/g vs BET for various Nb powders having arange of BETs. Each figure represents the measurement of CV/g for thepowders determined at a specific sinter temperature. As the figuresshow, the higher the sinter temperature the greater is the loss ofsurface area of the anode and there is also a general reduction in CV/gfor any particular powder sample as the sample is tested at highersinter temperatures (CV/g is proportional to the residual specificsurface area of the anode after sintering).

As illustrated by FIGS. 1 through 4, for any given sinter temperature,the CV/g achieved will have a relationship to the starting BET of thesample. As shown, low BET will produce low net CV/g and as BET rises theCV/g will rise. For materials having high BETs the degree of surfacearea loss during sintering is so great as to obliterate so much surfacearea that only a small fraction of the original high BET is left to beexpressed as CV/g after the sinter so CV/g drops off with the highestBETs. For this reason, the response of CV/g vs BET shows a maximum at aBET value that preserves the most net specific surface area aftersintering. In general, as shown in the figures, lower sinter temperaturewill achieve optimum CV/g with higher BET material; whereas, high sintertemperatures, which are very destructive to small, high BET particles,will achieve optimum CV/g with a lower BET powder.

There is generally an optimum BET for use at any given sintertemperature; and, the set of all optimum BETs form a response surfacerelative to the sinter temperatures. As shown in the figures, the CV/gis generally proportional to the BET, and CV/g shows a relationship tothe sinter temperatures. Thus, FIG. 5 shows the CV/g for each sintertemperature from FIGS. 1 through 3 plotted against the sintertemperature. FIG. 5 shows the CV/g that would be achieved at the 130° C.sinter to be on the order of about 61,000.

The preparation of FIG. 5 is based on an objective and mathematicallycorrect procedure for determining the maximum CV/g from each of theFIGS. 1 through 3. Because the response of CV/g vs BET in each of FIGS.1 through 3 is observed to exhibit a maximum, the requirement wasresolved by finding the maximum of the best functional fit to the datafor each figure. The actual response of CV/g to BET is a complexfunction of the variables; however, the Taylor Series expansion offunctions teaches that all functions can be approximated by the firstthree terms of the Taylor Series within a limited domain of theindependent variable (in this case BET). This amounts to approximatingthe function as a quadratic (F(x)=ax² +bx+c) valid within a limitedneighborhood of any selected value for x. This calculation is valid aslong as the values of x are within the neighborhood. The optimum BET ineach case was used as the center of the neighborhood of the BET domainso that the approximation is most valid for BET near this value; and, totherefore take the maximum of the quadratic fit to the data to be thebest estimate for the peak CV/g of the data in FIGS. 1 through 3. Forthis reason, a best fit of the data was performed in FIGS. 1 through 3to a quadratic function using the curve fitting tool in Microsoft Excelv 5.0 which generated the parabolic curves superimposed on the measureddata in FIGS. 1 through 3. The maximum of the fitted parabolas in FIGS.1 through 3 were used as the input data to make FIG. 5.

The set of maximum CV/g vs sinter temperature data in FIG. 5 was nextfitted to an exponential decay function using the curve fitting tool inMicrosoft Excel v 5.0. The reason for selecting exponential decay as thebest approximation to the response of maximum CV/g vs sinter temperatureis because, as shown in the figures, CV/g will decrease with increasingsinter temperature; however, CV/g can in principal never be lower than0.0 because the specific surface area cannot be made less than zero(cannot be negative). The exponential function which asymptoticallydecays to zero is the simplest functional form that can be used with thedata in FIG. 5 that does not predict negative CV/g. The best fit of anexponential curve as determined by Microsoft Excel v 5.0 was added tothe data in FIG. 5 and this allowed the calculation of the maximum CV/gthat would be achieved with a 1300° C. sinter temperature based upon allof the data from FIGS. 1 through 3 as explained above.

FIG. 4 is the actual data for the available Nb samples tested at the1300° C. sinter; however, it is seen in FIG. 4 that the data does notpeak because none of the samples had the optimum BET for the 1300° C.sinter. The data was fitted to the quadratic function just as was usedin FIGS. 1 through 3 and the result shown superimposed on FIG. 4 showsthe peak should exist following the observations of peaks in FIGS. 1through 3; and, the peak is shown to be a CV/g>55,000 and BET>1.7. It isreadily apparent that in the case of FIG. 4, the peak CV/g predicted byusing the same analysis used to make the data in FIG. 5 describes amaximum CV/g very close to the independently derived maximum estimatedby FIG. 5. The agreement between two separate determinations of themaximum CV/g at the 1300° C. sinter agree and make it clear that thematerials made with BET>1.7 (BETs on the order of 2 or more) willexhibit CV/g>55,000 (CV/g on the order of 60,000) when tested at 1300°C. sinter conditions.

                  TABLE 2                                                         ______________________________________                                        Example data used for FIGS. 1 through 4                                       1300  1300     1450   1450  1600 1600   1750 1750                             BET   CV/g     BET    CV/g  BET  CV/g   BET  CV/g                             ______________________________________                                        0.8   30,302   0.8    22,757                                                                              0.8  14,433 0.8  7,972                            0.8   30,423   0.8    22,982                                                                              0.8  14,754 0.8  8,517                            1.16  45,440   1.16   29,916                                                                              1.16 16,495 1.16 7,785                            0.96  40,797   0.96   29,868                                                                              0.96 18,480 0.96 9,958                            0.96  35,350   0.96   27,959                                                                              0.96 17,742 0.96 9,611                            0.96  40,235   0.96   30,147                                                                              0.96 18,707 0.96 9,989                            0.96  35,481   0.96   27,667                                                                              0.96 17,977 0.96 9,611                            ______________________________________                                    

EXAMPLE 10

The effects of oxygen on niobium powders were studied. Five samples offlaked niobium powder (prepared as in Example 5) each weighing 1 pound,were tested. One of the samples was a control and the four remainingsamples were processed in such a manner as to increase the oxygencontent in the niobium powder. In particular, the four samples were heattreated in a furnace at 900° C. for 30 minutes. The powders were thenpassivated in air in a manner similar to the air passivation discussedin the proceeding examples. Then, one of the four samples was removedand the three remaining samples heat treated and passivated again in thesame manner described above. As before, one of these three samples wasthen remove and the procedure was repeated again with the two remainingsamples. Afterward, again one of the samples was removed and the finalremaining sample was again heat treated and passivated as before. Thus,there were five samples prepared wherein either 0, 1, 2, 3, or 4 cyclesof heat treatment were preformed. Prior to testing for the oxygencontent in each of these samples, the samples were passed individuallythrough a 40 mesh screen.

The powders were then agglomerated and sintered at various temperaturesand formed into anodes based on three different forming voltages asindicated in Table 3. The results of the DC leakage are also set forthin Table 3. As can be seen from the results in Table 3 and in FIGS. 6and 7, the DC leakage gradually decreased as the oxygen content in theniobium increased. The decrease in DC leakage was especially apparentwith lower forming voltages such as 30 and 50 volts.

                  TABLE 3                                                         ______________________________________                                        Data showing effect of O.sub.2 on na/CV at 30, 50 and 60 Volts                          1300    1450        1600  1750                                      Oxygen    na/CV   na/CV       na/CV na/CV                                     ______________________________________                                        30 Vf                                                                         2725      4.47    1.86        0.89  0.47                                      4074      3.96    1.41        0.62  0.47                                      4870      3.49    1.29        0.58  0.45                                      5539      2.7     1.04        0.55  0.45                                      6499      2.38    0.95        0.54  0.45                                      8909      2.25    0.88        0.57  0.54                                      50 Vf                                                                         2725      4.31    3.07        1.84  1.08                                      4074      4.47    2.55        1.46  1.01                                      4870      3.96    2.51        1.42  0.99                                      5539      3.26    2.21        1.29  0.97                                      6499      3.5     2.09        1.23  0.97                                      8909      3.85    2.02        1.26  0.97                                      60 Vf                                                                         2725      22.16   25.06       28.64 27.08                                     4074      19.78   24.07       28.51 28.78                                     4870      19.11   24.71       28.51 27.67                                     5539      17.84   21.75       26.62 27.37                                     6499      17.88   22.37       24.88 25.69                                     8909      25.2    29.67       33.2  28.99                                     ______________________________________                                    

EXAMPLE 11

The effect of phosphorus on niobium powder was then examined. Sixsamples of niobium powder prepared in a manner like Example 5 weretested. One of the samples was used as a control and the remaining fivesamples had sufficient phosphoric acid added to achieve phosphoruslevels of 5 ppm, 10 ppm, 30 ppm, 100 ppm, and 500 ppm respectively. Thephosphoric acid was added as a diluted solution with 150 ml of deionizedwater. The phosphoric acid solution and powder were mixed and thesamples dried in a vacuum oven. After drying, the samples wereindividually blended and tested for phosphorus levels. The results areset forth in Table 4. As can be seen in Table 4 and FIGS. 8 and 9, therewas a small effect caused by phosphorus doping and it was noticed thathigher amounts of phosphorus doping did not necessarily improve theproperties of DC leakage.

                  TABLE 4                                                         ______________________________________                                        P doped Niobium samples data                                                  doped P (ppm)                                                                            anode P (ppm)                                                                              CV/g     na/CV                                        ______________________________________                                                                (1300° C.)                                                                      (1300° C.)                            16         13           33009    22.96                                        26         13           33614    21.77                                        69         100          33676    19.53                                        200        58           33915    21.83                                        400        204          34213    20.65                                                                (1450° C.)                                                                      (1420° C.)                            16         0            24095    25.06                                        26         20           24375    23.4                                         62         72           24459    24.33                                        200        50           25348    26.09                                        400        339          25664    24.69                                                                 (1600° C.)                                                                     (1600° C.)                            16         0            15757    25.51                                        26         0            15974    24.82                                        62         0            16131    24.57                                        200        56           16736    25.83                                        400        415          17011    27.18                                                                (1750° C.)                                                                      (1750° C.)                            16                      8575     16.39                                        26                      9176     16.69                                        62                      9315     17.35                                        200                     9551     16.54                                        400                     9670     18.74                                        ______________________________________                                    

EXAMPLE 12

The effects of nitrogen doping were studied.

Nitrogen Doping Example Pedigree:

Lots of Samples 60-, 25-, 55-, and 46-14XX

Thrice melted electron beam produced niobium ingot was hydrided byheating the ingot in a vacuum of 10⁻⁴ torr to 850° C. for 120 minutes.The vacuum was replaced by hydrogen gas purge at 21 kPa for a sufficienttime to embrittle the ingot. The vacuum was then pumped down to 28" Hgand backfilled with argon to -5" Hg. The pressure was maintained untilthe temperature, as measured by work thermocouple, stabilized. Air wasgradually introduced in increasing pressure such that the worktemperature did not rise.

Size reduction was accomplished by a Granutec crusher to reduce themaximum particle to that which would pass through a No. 20 sieve screen.Further reduction was accomplished by repeated processing in an impactmill by Vortec Products operated at 20,000 rpm until the material met aminimum of 95% passage through a No. 325 sieve screen (equivalent to 44micron particle size).

Hydrogen was removed by heating the particles to 850° C. in a vacuumuntil pressure was no longer affected by hydrogen being emitted from theparticles to provide niobium metal angular powder having a Scott Densityof 42 and a BET of 0.22 m² /g.

The powder was then milled in a 30 S Union Process Attritor Mill

Media: 1,000 lbs. 3/16" SS media

80 lbs. Niobium powder

Duration: 4 hours, 30 minutes

13 gallons of ethanol

130 rpm

The material was then vacuum filtered to remove residual ethanol andthen washed with deionized water until no ethanol odor was noted andvacuum filtered to remove residual water. The wet powder was then washedin a slurry of hydrochloric acid (250 ml/lb.), hydrofluoric acid (6ml/lb.) and de-ionized water (250 ml/lb.) to remove metalliccontamination. The powder was then washed with deionized water and driedin air at 80° C.

The dried powder was hydrided a second time by exposure to hydrogen at20.7 kPa and 850° C. to provide an embrittled flake which was cooled andreduced in size by an impact mill by Vortec Products to achieve a Scottdensity of 21.4.

The reduced size flaked powder was then agglomerated by heating in avacuum at 1250° C. for 60 minutes to provide an agglomerated mass whichwas reduced in size to agglomerated particles of a maximum of a No. 50sieve by a jaw crusher.

The powder was then deoxidized by blending with 4% w/w magnesium metal.The following thermal treatment was then executed in a retort furnace.

Reduce pressure to 100 microns.

Backfill with argon to 800 torr and 800° C. for 2 hours

Cool to room temperature

Passivate by increasing air content over 30 cycles of 2 minutes eachwherein the system pressure progresses from high vacuum to atmospheric.

The material was then washed with a mixture of deionized ice (500g/lb.), hydrofluoric acid (4 ml/lb.) and nitric acid (250 ml/lb.). Thepowder was then rinsed with deionized water and air dried at 80° C.(14B1)

Nitrogen doping procedure for samples 14XXX.

The niobium powder was then mixed with 4% magnesium powder and heated to800° C. in argon and held at that temperature for 60 minutes. The vesselwas evacuated and allowed to cool to 70° C. 130 Torr of N2 gas was heldon the vessel and the temperature raised to 500° C. and held for 60minutes under the argon atmosphere. The product was cooled to 40° C. andadmitted to air gradually using standard passivation techniques.

Lots of samples 60-, 25-, 55, and 46-39XX

Thrice melted electron beam produced niobium ingot was hydrided byheating the ingot in a vacuum of 10⁻⁴ torr to 850° C. for 120 minutes.The vacuum was replaced by hydrogen gas purge at 21 kPa for a sufficienttime to embrittle the ingot. The vacuum was then pumped down to -28" Hgand backfilled with argon to -5" Hg. The pressure was maintained untilthe temperature, as measured by work thermocouple, stabilized. Air wasgradually introduced in increasing pressure such that the worktemperature did not rise.

Size reduction was accomplished by a Granutec crusher to reduce themaximum particle to that which would pass through a No. 20 sieve screen.Further reduction was accomplished by repeated processing in an impactmill by Vortec Products operated at 20,000 rpm until the material met aminimum of 95% passage through a No. 325 sieve screen (equivalent to 44micron particle size).

Hydrogen was removed by heating the particles to 850° C. in a vacuumuntil pressure was no longer affected by hydrogen being emitted from theparticles to provide niobium metal angular powder having a Scott Densityof 42 and a BET of 0.22 m² /g. The product was then classified at +8microns utilizing a Vortec Classifier.

The powder was then milled in a 30 S Union Process Attritor Mill

Media: 1,000 lbs. 3/16" SS media

80 lb. Niobium powder

Duration: 6 hours.

13 gallons ethanol

130 rpm

The material was then vacuum filtered to remove residual ethanol thenwashed with deionized water until no ethanol odor was noted and vacuumfiltered to remove residual water. The wet powder was then washed in aslurry of hydrochloric acid (250 ml/lb.), hydrofluoric acid (6 ml/lb.)and de-ionized water (250 ml/lb.) to remove metallic contamination. Thepowder was then washed with de-ionized water and dried in air at 80° C.

The dried powder was hydrided a second time by exposure to hydrogen at20.7 kPa and 850° C. to provide an embrittled flake which was cooled andreduced in size by an impact mill by Vortec Products to achieve a Scottdensity of 21.4.

The reduced size flaked powder was then agglomerated by heating in avacuum at 1200° C. for 60 minutes to provide an agglomerated mass whichwas reduced in size to agglomerated particles of a maximum of a No. 50sieve by a jaw crusher. It was then thermally agglomerated a second timeto 1225° C. for 60 minutes. After cooling the furnace down to a targetedlevel (see following table), nitrogen was pumped in at a rate of 5 SCFHfor the specified time under vacuum. Material was then passivated bygradually increasing the operating pressure with air over a forty minuteperiod until atmospheric pressure was restored.

    ______________________________________                                        Temperature (° C.)                                                                   Soak Time (minutes)                                                                         Nitrogen (ppm)                                    ______________________________________                                        752           90            4,155                                             932           120           1,580                                             752           60            830                                               752           120           776                                               ______________________________________                                    

This material was then crushed a second time to reduce the maximumobserved particle to one which would pass through a No. 50 sieve.

The powder was then deoxidized by blending with 4% w/w magnesium metalthen doped with nitrogen. The following thermal treatment was thenexecuted in a retort furnace.

Reduce pressure 100 microns.

Backfill with argon to 800 torr and 800° C. for 2 hours

Cool to 70° C. add 500 torr nitrogen

Heat to 500° hold for 60 minutes and cool to 40° C.

Passivate by increasing air content over 30 cycles of 2 minutes eachwherein the system pressure progresses from high vacuum to atmospheric.

The material was then washed with a mixture of deionized ice (500g/lb.), hydrofluoric acid (4 ml./lb.) and nitric acid (250 ml./lb.). Thepowder was then rinsed with deionized water and air dried at 80° C.Table 5 sets forth the analysis of the powder.

                  TABLE 5                                                         ______________________________________                                        Results                                                                                C (ppm) O (ppm)    N (ppm)                                                                              H (ppm)                                    ______________________________________                                        14       251     2,202      273    30                                         14N2     289     1,231      3,399  41                                         14N3     292     1,149      4,409  40                                         39N1     204     1,530      2,877  48                                         ______________________________________                                    

The results of these experiments are set forth in Tables 6 and 7, whichare also plotted in FIGS. 10-21. As can be seen, the nitrogen levels inthe niobium contributed to lowering the DC leakage and at highersintering temperatures of the anode, DC leakage was reduced evenfurther, especially when using low forming voltages.

Capacitor anodes were prepared in a similar manner as the earlierexamples.

                                      TABLE 6                                     __________________________________________________________________________    Niobium samples prepared to compare effects of nitrogen doping during         deox on the DC leakage (na/CV)                                                at 35 and 50 volts formations                                                 __________________________________________________________________________    Forming                                                                       Voltage                                                                             35 v                                                                    Sintering                                                                     Temperature                                                                         1300° C.     1450° C.                                     sample ID                                                                           25-14B1                                                                            55-39N1                                                                            46-14N2                                                                            55-14N3                                                                            25-14B1                                                                            55-39N1                                                                            46-14N2                                                                            55-14N3                              __________________________________________________________________________    CV/g  30,473                                                                             31,042                                                                             28,464                                                                             28,272                                                                             17,109                                                                             20,588                                                                             18,776                                                                             18,983                               na/CV 8.44 5.03 4.20 4.46 3.70 1.94 0.99 0.92                                 N2    273  2877 3399 4409 273  2877 3399 4409                                 O2    2202 1530 1231 1149 2202 1530 1231 1149                                 __________________________________________________________________________    Forming                                                                       Voltage                                                                             50 v                                                                    Sintering                                                                     Temperature                                                                         1300° C.     1450° C.                                     sample ID                                                                           25-14B1                                                                            55-39N1                                                                            46-14N2                                                                            55-14N3                                                                            25-14B1                                                                            55-39N1                                                                            46-14N2                                                                            55-14N3                              __________________________________________________________________________    CV/g  29,243                                                                             30,152                                                                             27,633                                                                             27,514                                                                             16,941                                                                             20,495                                                                             18,574                                                                             18,744                               na/CV 7.74 4.57 4.70 6.86 2.90 2.22 2.36 1.93                                 N2    273  2877 3399 4409 273  2877 3399 4409                                 O2    2202 1530 1231 1149 2202 1530 1231 1149                                 __________________________________________________________________________

                  TABLE 7                                                         ______________________________________                                        Niobium DC Leakage, oxygen and CV/g versus nitrogen added during              heat treatment                                                                               N2      O2    DCL   Vf                                         sample CV/g    (ppm)   (ppm) (na/cv)                                                                             (volts)                                                                             Temp (° C.)                   ______________________________________                                        60-39N1                                                                              33,111  3190    1877  6.49  35    1300                                 60-39N2                                                                              32,668  561     2248  5.68  35    1300                                 60-39N3                                                                              32,812  544     2172  5.87  35    1300                                 60-14B 29,348  286     1691  9.56  35    1300                                 60-39A 32,198  489     2134  5.17  35    1300                                 60-39N1                                                                              21,330  3190    1877  1.65  35    1450                                 60-39N2                                                                              19,879  561     2248  3.31  35    1450                                 60-39N3                                                                              19,591  544     2172  2.75  35    1450                                 60-14B 17,138  286     1691  3.85  35    1450                                 60-39A 19,492  489     2134  3.15  35    1450                                 60-39N1                                                                              32,224  3190    1877  7.60  50    1300                                 60-39N2                                                                              31,860  561     2248  6.05  50    1300                                 60-39N3                                                                              31,991  544     2172  6.49  50    1300                                 60-14B 28,411  286     1691  15.12 50    1300                                 60-39A 31,570  489     2134  6.84  50    1300                                 60-39N1                                                                              21,327  3190    1877  4.70  50    1450                                 60-39N2                                                                              19,853  561     2248  4.67  50    1450                                 60-39N3                                                                              19,598  544     2172  4.02  50    1450                                 60-14B 17,023  286     1691  8.96  50    1450                                 60-39A 19,428  489     2134  5.00  50    1450                                 ______________________________________                                    

Other embodiments of the present invention will be apparent to thoseskilled in the art from consideration of the specification and practiceof the invention disclosed herein. It is intended that the specificationand examples be considered as exemplary only, with a true scope andspirit of the invention being indicated by the following claims.

What is claimed is:
 1. Niobium powder having a nitrogen content of atleast about 300 ppm, wherein said niobium powder has a BET surface areaof at least about 1.0 m² /g.
 2. The niobium powder of claim 1, whereinsaid nitrogen content is at least about 400 ppm.
 3. The niobium powderof claim 1, wherein said nitrogen content is at least about 500 ppm. 4.The niobium powder of claim 1, wherein said nitrogen content is fromabout 300 to about 5,000 ppm.
 5. The niobium powder of claim 1, whereinsaid nitrogen content is from about 500 ppm to about 4,000 ppm.
 6. Theniobium powder of claim 1, wherein said nitrogen content is from about500 ppm to about 3,500 ppm.
 7. The niobium powder of claim 1, whereinsaid nitrogen content is from about 500 ppm to about 3,000 ppm.
 8. Theniobium powder of claim 1, wherein said nitrogen content is from about1,500 ppm to about 5,000 ppm.
 9. The niobium powder of claim 1, whereinwhen said powder is formed into an electrolytic capacitor anode, saidanode has a DC leakage which is less than an electrolytic capacitoranode formed from niobium powder having substantially no nitrogen. 10.The niobium powder of claim 9, wherein the DC leakage is decreased about50% or less compared to the electrolytic capacitor anode formed fromniobium powder having substantially no nitrogen.
 11. The niobium powderof claim 1, wherein the DC leakage is decreased about 25% or lesscompared to the electrolytic capacitor anode formed from niobium powderhaving substantially no nitrogen.
 12. The niobium powder of claim 1,wherein the niobium powder comprises flaked niobium powder.
 13. Theniobium powder of claim 12, wherein said powder has a BET surface areaof at least about 2.0 m² /g.
 14. The niobium powder of claim 12, whereinsaid powder has a BET surface area of from about 1.0 to about 5.0 m² /g.15. The niobium powder of claim 12, wherein said powder has a BETsurface area of from about 2.0 to about 5.0 m² /g.
 16. The niobiumpowder of claim 12, wherein said powder has a Scott Density of less thanabout 35 g/in³.
 17. The niobium powder of claim 1, wherein when saidpowder is formed into an electrolytic capacitor anode, said anode has acapacitance of from about 30,000 CV/g to about 61,000 CV/g.
 18. Acapacitor made from the niobium powder of claim
 12. 19. The niobiumpowder of claim 1, wherein said niobium powder comprises nodular,angular niobium powder, or combinations thereof.
 20. The niobium powderof claim 1, having a BET surface area of from about 2.0 to about 5.0 m²/g.
 21. A capacitor prepared from a formulation comprising the niobiumpowder of claim
 1. 22. The capacitor of claim 21, wherein said powder issintered at a temperature of from about 1200° C. to about 1750° C. 23.The capacitor of claim 21, wherein said powder is sintered at atemperature of from about 1200° C. to about 1450° C.
 24. The capacitorof claim 21, wherein said powder is sintered at a temperature of fromabout 1250° C. to about 1350° C.
 25. A capacitor prepared from aformulation comprising the niobium powder of claim
 5. 26. A capacitorprepared from a formulation comprising the niobium powder of claim 8.27. The niobium powder of claim 1, having a phosphorus level of lessthan about 400 ppm.
 28. The capacitor of claim 21, wherein saidcapacitor is formed at a voltage of about 50 volts or less.
 29. Thecapacitor of claim 21, wherein said capacitor has a DC leakage of lessthan about 5.0 na/CV.
 30. The capacitor of claim 21, wherein saidcapacitor has a DC leakage of from about 5.0 na/CV to about 0.50 na/CV.31. The capacitor of claim 21, further comprising a niobium oxide filmon a surface thereof.
 32. The capacitor of claim 31, wherein said filmcomprises niobium pentoxide film.
 33. The niobium powder of claim 1,further comprising an oxygen content of at least about 2,000 ppm oxygen.34. Niobium powder having a nitrogen content of at least about 300 ppmand having a phosphorous level of less than about 400 ppm.
 35. Acapacitor anode comprising niobium powder having a nitrogen content ofat least about 300 ppm, wherein said capacitor anode is formed at avoltage of about 50 volts or less.
 36. The niobium powder of claim 1,having a BET surface area of from about 1.0 to about 5.0 m² /g.
 37. Anelectrolytic capacitor anode comprising niobium powder having a nitrogencontent of at least about 300 ppm, wherein said anode has a capacitanceof from about 30,000 CV/g to about 61,000 CV/g.